Electric arc welding with continuously fed electrodes is performed with various techniques. These techniques comprise submerged arc welding, where welding is performed under a protective layer of flux, and gas metal arc welding where welding is performed under a protective gas shield.
In order to increase the productivity it has been suggested to use more than one electrode in the welding process. Known such welding processes are tandem and twin welding methods, which are used to increase the production rate when welding.
Tandem welding is a welding technology where two independent electrodes are arranged to perform welding in a common weld pool.
Each of the two electrodes are connected to a separate power source such that the welding conditions comprising current and voltage can be controlled independently for the electrodes. The electrodes can be mounted in various settings, such as for instance with the electrodes positioned shifted out in a transverse direction with respect to a welding direction or being positioned at a distance from each other in the welding direction. In the event the electrodes are shifted out in the transverse direction, they may be positioned side by side. This is used for surface welding or specific joints where a wide joint is needed. Side by side welding leads to lower penetration and more width.
Alternatively the electrodes are positioned at a distance from each other in the welding direction. In this event, the first electrode in the direction of welding is normally referred to as a leading electrode, while the second electrode is normally referred to as a trailing electrode. Normally the leading electrode and the trailing electrodes serve different roles in the welding process. It is for instance known to control the leading electrode such that a desired degree of penetration is obtained, while the trailing electrode controls weld bead appearance, contour and fill.
Tandem welding enables increased deposition rates and therefore improves economy of welding. Tandem welding also enables improved weld quality due to the possibility of assigning the leading and trailing electrode different tasks. In order to allow the different electrodes to be assigned different tasks the electrodes should preferably be sufficiently close to enable both electrodes to operate in a common single weld pool. In the event the electrodes are separated too far from each other, the weld pool generated by the leading electrode may solidify before the trailing electrode has reached the puddle. In this event, the two electrodes more or less perform the task of two consecutive welding passes.
Twin welding is similar to tandem welding, with the difference that the electrodes are connected to the same power source.
Another way to improve the deposition rate is to add one or more consumable electrodes that melt without formation of arcs. These electrodes are sometimes referred to as cold wires, whereas electrodes arranged to generate arcs are referred to as hot wires. A cold wire is continuously fed into the arcs of one or more hot wires, where the cold wire is melted by excess energy generated by said hot wire(s). A current may be transferred through a portion of a cold wire for heating thereof.
The introduction of cold wire material into the weld pool may lead to improved control of the composition of the weld alloy, which may lead to improved welds. Feeding of cold wire material into the weld pool may lead to an increase in productivity of up to 100% with optimized welding parameters. In other words, a cold wire allows for higher deposition rates without increasing the heat input.
A problem associated with cold wires is that they sometimes increase the welding process instability when the cold wire is not melted at an even pace. The may also strike the parent material through the melt pool. This can cause weld defects and inclusions in the weld metal of unmelted cold wire material. Also, the amount of cold wire that can be added to the weld pool is limited by the amount of excess energy generated by the hot wires.
It is a first object of the invention is to provide an efficient electric arc welding method, which makes possible an increased production rate.
A second object of the invention is to provide an electric arc welding arrangement, which makes possible an increased production rate and more efficient welding.